Method for selectively applying solder mask

ABSTRACT

A method is disclosed for applying solder mask to an area respective of a plurality of receiving pads on a Printed Circuit Board. The solder mask can be at least partially removed during the component removal process exposing the conductor, via and annular ring. The exposed conductor, via and annular ring can cause defects and unreliable solder joints should the solder flow along the conductor and into the via. A small stencil can be used to apply solder mask in a desirable pattern to a selective area of a PCB. Alternatively, an elastomeric transferring apparatus can be used. Various methods can be included to assist in aligning the applicator to the desired area such as an overlaid, dual imaging system.

RELATED PATENT APPLICATIONS

[0001] This application is a divisional of pending application Ser. No.09/659,209 filed on Sep. 11, 2000. Application Ser. No. 09/659,209 isincorporated herein by reference.

FIELD OF THE INVENTION

[0002] This invention relates in general to the field of reworkprocesses for surface mount technology, and in particular a method forreapplying a solder mask material for areas of solder mask that areremoved during the component rework process such as those for a BallGrid Array (BGA).

BACKGROUND OF THE INVENTION

[0003] Printed Circuit Boards (PCB's) are manufactured using variousadditive and subtractive process. The circuitry is generated by startingwith a core panel of copper laminated to at least one side of a corematerial. Generally, the copper is laminated to both sides of the corematerial. The copper is generally chemically etched usingphotodeveloping processes and various chemicals to create the circuitry.Several cores are then laminated to create a multilayer PCB. The severalcores are drilled in predetermined locations and later plated to createelectro-mechanical connections between the circuits of the variouslayers, commonly referred to as vias. Solder resist or mask is appliedto each outer layer of the laminated PCB, over the bare copper.

[0004] Solder mask is required to control the flow of solder and containthe solder to the desired solder pads, respective to each component.This is particularly critical for Ball Grid Array (BGA) devices, wherebythe solder pads are attached to a via. The solder is preapplied to theBGA in predefined volumes. During reflow, the solder liquifies and flowsaround the receiving pads on the PCB. Should the solder mask be removed,when attaching a replacement component, the solder can flow from thereceiving pad down the via, resulting in a defect. Defects can includeopen circuits, mis-registered components, shorts, and reliability risks.

[0005] Conventional methods for reworking surface mount components,require heat to remove the component previously reflowed to a PrintedCircuit Board (PCB). The heating process degrades the adhesion betweenthe solder mask and the bare copper circuitry. Further, any residualsolder needs to be removed from the receiving pads. This is generallycompleted by placing solder wick (pre-fluxed, braided copper strandswoven into a ribbon) against the pads and heating the wick using asoldering iron. The solder wick is known to abrade and remove the soldermask from the PCB. Solder may also flow under the resist and cause theresist to lift from the copper.

[0006] The solder mask process is generally applied to the PCB duringthe PCB fabrication process. The process utilized phototooling andphotodeveloping systems. These systems require expensive equipment and aflat surface. These two factors make replacement of the solder mask on apopulated assembly at an assembly shop impractical.

[0007] It is known to use micro stencils, such as U.S. Pat. No.5,107,759, Omori, et al. dated Apr. 28, 1992 to apply solder to solderpads for replacement of solder during the rework process.

[0008] It is known to use look up/look down prism systems during thealignment process of various solder printing and component placementprocesses. One known example is taught by Freeman, U.S. Pat. No.4,924,304, issued May 8, 1990.

[0009] Thus, what is necessary is a low cost and efficient method andapparatus for replacing solder mask around pads on a component or on asubstrate, where the solder mask has been removed during a reworkprocess.

SUMMARY OF THE INVENTION

[0010] One aspect of the present invention is to provide a low cost toolfor replacing solder mask or a similar type material onto exposedmetallization of a component or PCB.

[0011] A second aspect of the present invention is the ability to applya pattern of solder mask to a desired area, leaving the desired solderpads exposed.

[0012] A third aspect of the present invention is a solder maskapplicator to apply the pattern of solder mask to the desired area.

[0013] A fourth aspect of the present invention is the use of a foil todefine the desired pattern.

[0014] A fifth aspect of the present invention is the use of a rubberstamp to define the desired pattern.

[0015] A sixth aspect of the present invention is the inclusion of amechanism to aid in registration of the pattern on the solder maskapplicator and the receiving site.

[0016] A seventh aspect of the present invention is the use of a prism,look up/look down system to align the pattern of the solder maskapplicator and the receiving site.

[0017] An eighth aspect of the present invention is the use of apredetermined depth, solder mask reservoir to apply the solder mask tothe solder mask applicator.

[0018] A ninth aspect of the present invention is the incorporation ofthe present invention into a component rework apparatus, preferably onewhich comprises a means to provide alignment, a means to controlrelational proximity between the selective solder mask applicator andthe plurality of receiving pads, and a controllable means to apply heatas required.

[0019] A tenth aspect of the present invention is the inclusion of ameans to at least partially automate the processes described herein,such as including a means to apply a predetermined set of instructionsto an automated means.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 is a top view of a PCB in the original or desirable statewith solder mask in tact.

[0021]FIG. 2 is a top view of a PCB in a non-desirable state afterrework, illustrating removed solder mask.

[0022]FIG. 3 is a top view of a solder mask applicator using a stencil.

[0023]FIG. 4 is an isometric view of a solder mask applicator using arubber stamp.

[0024]FIG. 5 is an isometric view of a solder mask reservoir for use inconjunction with the solder mask applicator of FIG. 4.

[0025]FIG. 6 is an isometric view illustrating one potential alignmentapparatus using an imaging system.

[0026]FIG. 7 is a flow diagram illustrating a process of applying soldermask to a PCB using a stencil.

[0027]FIG. 8 is a flow diagram illustrating a process of applying soldermask to a PCB using a transfer mechanism such as a rubber stamp.

DETAILED DESCRIPTION OF THE INVENTION

[0028]FIG. 1 illustrates a top view of a section of a Printed CircuitBoard (PCB) 10 shown in a preferred state. The Figure illustrates soldermask 12 adhered to the surface of the PCB 10. A Ball Grid Array (BGA)receiving pad solder mask etch-back 22 is included in the layout of thePCB, whereby the BGA receiving pad solder mask etch-back 22 is used toavoid placing solder mask 12 over a plurality of BGA receiving pads 14.The solder mask 12 is preferred to cover a plurality of vias 16,respective plurality of annular rings 18, and respective plurality ofconductors 20. The solder mask 12 provides a barrier between a pluralityof receiving pads 14 and the plurality of vias 16, respective pluralityof annular rings 18, and respective plurality of conductors 20.Component receiving pads 24 and respective Component solder masketch-back 26 is further illustrated.

[0029]FIG. 2 illustrates a section of a PCB 10 shown after removal of aBGA component (not shown). During the component removal process andsubsequent removal of residual solder on the plurality of BGA receivingpads 14, solder mask 12 can be removed from at least a portion of theplurality of vias 16, respective plurality of annular rings 18, andrespective plurality of conductors 20 as shown by the non-desirableexposed areas of copper 30. FIG. 2 further illustrates a component 28adjacent to the plurality of BGA receiving pads 14 as one would find ona commonly assembled PCB 10. The component 28 limits the availableplanar surface of the PCB, thus not allowing one to replace the soldermask 12 using the original photo-developing application process. Theexposed portion of the plurality of vias 16, respective plurality ofannular rings 18, and respective plurality of conductors 20 provides apath for solder to flow during the reflow process. The solder is appliedto the BGA receiving pads 14 by any of known methods including the useof the solder spheres applied to the connecting side of the BGA. It ispossible and known that the solder can flow along the conductor 20 tothe annular ring 18 and down the via 16, resulting in a defect. Further,the variation in resulting joint geometry (not shown) can impactreliability. What is desired is a means to re-apply solder mask to aspecific area in a desirable pattern to cover the plurality of vias 16,respective plurality of annular rings 18, and respective plurality ofconductors 20, while exposing the BGA receiving pads 14.

[0030]FIG. 3 is top view of a selective solder mask applicator 40. Theselective solder mask applicator 40 comprises a masking means 42, aplurality of openings 44, and a stiffening means 46. The selectivesolder mask applicator 40 can optionally include a means for alignment48. The masking means 42 can be of an etched metal foil such as astencil, a silk screen and respective photodeveloped masking, and thelike. All of these and other known masking means are well known to thoseskilled in the art. The stiffening means 46 would be respective to thecomposition of the masking means 42. The stiffening means 46 can be anextruded or cast frame, bent material if metal is used, and the like.One example of a means for alignment 48 would be to half etch a cross onside of the masking means 42 which contacts the PCB 10. The plurality ofopenings 44, would be of a pattern which at least covers the pluralityof vias 16, respective plurality of annular rings 18, and respectiveplurality of conductors 20, while exposing the BGA receiving pads 14.The user would align the selective solder mask applicator 40 proximatethe BGA receiving pads 14, squeegee solder mask 12 (or similar material)in its liquid form across the masking means, remove the selective soldermask applicator 40 from the PCB 10, and lastly cure the applied soldermask 12. Once the BGA site is prepared, the BGA can be replaced withconfidence.

[0031]FIG. 4 is an isometric view of an alternate selective solder maskapplicator 50. The alternate selective solder mask applicator 50comprises a formed section of elastomeric material 52. The elastomericmaterial 52 can be coupled to a backing material 58 to add rigidity. Theelastomeric material 52 is formed to provide a transferring surface 54and a plurality of cavities 56, similar to that of a rubber stamp. Thecombination of a transferring surface 54 and plurality of cavities 56creates a desirable pattern to transfer solder mask which at leastcovers the plurality of vias 16, respective plurality of annular rings18, and respective plurality of conductors 20, while exposing the BGAreceiving pads 14. Solder mask 12 would be transferred from a reservoir(not shown) to the transferring surface 54 of the alternate selectivesolder mask applicator 50. The alternate selective solder maskapplicator 50 is held by a holding means 60 and located proximate theBGA receiving pads. The solder mask 12 would be deposited onto the PCB10. The solder mask 12 would be cured. Once the BGA site is prepared,the BGA can be replaced with confidence.

[0032] The alternate selective solder mask applicator 50 can bemanufactured using any known technology similar to that of making a felttipped marker or a rubber stamp. The transferring surface 54 may furtherinclude texturing or smaller recesses to aid in the transferring processof the solder mask 12.

[0033] Since the surface of the PCB 10 is known to be non-planar, it isadvantageous to use a compliant, elastomeric material 52 as atransferring mechanism.

[0034]FIG. 5 is an isometric view of a sample solder mask reservoir 70.A solder mask reservoir can be defined as anything capable of holdinguncured solder mask 76 in a manner capable of applying a desiredthickness to the transferring surface 54 of the selective solder maskapplicator 50. The figure illustrates a reservoir block 70 which can bemachined to include a first top surface 74 and a second, recessedsurface or trough 72. The trough can be milled to a desirable depth,whereby uncured solder mask 76 can be deposited in the trough 72 andleveled by passing 80 a squeegee 78 or similar across the top orleveling surface 74 of the reservoir block 70. The solder mask can be ofsolder mask material, chip bonder, epoxy, and the like. Once the soldermask is applied, it can then be cured a manner respective to theselected material.

[0035]FIG. 6 is an isometric view of a sample solder mask applicator 50in relation with a plurality of electronic pads 14 located on a sectionof PCB 10. The figure illustrates a method of aligning the sample soldermask applicator 50 and the plurality of electronic pads 14 located on asection of PCB 10. Although the figure illustrates one method, it can berecognized that numerous known means of aligning the sample solder maskapplicator 50 and the plurality of electronic pads 14 located on asection of PCB 10 can be used. In accordance with a preferredembodiment, an alignment system utilizes an image splitter 90 tosuperimpose a first image 92 of the sample solder mask applicator 50 anda second image 94 of the plurality of electronic pads 14 to get anoverlaid image 96. The alignment system would include a mechanism toadjust the relation of the applicator 50 and the pads 14, preferablyincluding a coarse and a fine adjustment means. Once aligned, theapparatus would further comprise a means to remove the imaging systemfrom between the applicator 50 and the pads 14, then a means to bringthe applicator 50 and the pads 14 in contact without affecting therelational alignment of the two. The apparatus would also provide ameans to separate the applicator 50 and the pads 14, thus transferringthe solder mask 76 (applied to the transferring surface 54) to thesurface of the PCB 10, without applying solder mask 76 to the pluralityof electronic pads 14.

[0036]FIG. 7 is a flow diagram illustrating a method 100 for applyingselective solder mask to a PCB using a stencil. In a first step 110, thecomponent (not shown) is removed from the assembled PCB 10. The removalprocess can be of any known removal methods, such as convection reworkprocesses, where hot air is blown through a nozzle onto the component.In a second step 112, any residual solder (not shown) is removed fromthe plurality of electronic pads 14. The residual solder can be removedby applying conductive heat through solder wick to transfer the residualsolder to the solder wick. This process is required to maintain arepeatable volume of solder within the resulting solder joints uponcompletion of the component replacement process. It can be recognizedthat there are numerous methods for removing components and residualsolder which should not impose limitations to the present invention. Ina third step 114, a selective solder mask applicator 40 is located to adesirable location with respect to the plurality of electronic pads 14.This can be accomplished by any number of methods, including thoseillustrated in FIG. 6, visual alignment through the apertures, and thelike. In a fourth step 116, uncured solder mask would be applied to theexposed surface of the selective solder mask applicator 40; the soldermask is passed across the plurality of apertures 44 using a squeegee orsimilar; and thus be transferred in a desired, selective pattern to thePCB 10. In a fifth step 118, the selective solder mask applicator 40 isremoved from the PCB 10, leaving the desired, selective pattern ofuncured solder mask on the PCB 10. In a sixth step 120, the desired,selective pattern of uncured solder mask is cured using a curing methodrespective to the said solder mask. One example would be to apply heatto a heat curing material such as Loctite™ chip bonder. In a seventhstep 122, a replacement component is attached to the PCB using arespective attachment process. For the example of a BGA, the componentcan be dipped into a predetermined thickness of flux, placed onto theplurality of receiving pads 14, and heated using convection heating tocause solder balls which are pre-attached to the component to becomeliquidous and create electromechanical interconnections between thecomponent and the plurality of electronic pads 14.

[0037]FIG. 8 is a flow diagram illustrating a method 200 for applyingselective solder mask to a PCB using a transfer mechanism such as arubber stamp. In a first step 210, the component (not shown) is removedfrom the assembled PCB 10. The removal process can be of any knownremoval methods, such as convection rework processes, where hot air isblown through a nozzle onto the component. In a second step 212, anyresidual solder (not shown) is removed from the plurality of electronicpads 14. The residual solder can be removed by applying conductive heatthrough solder wick to transfer the residual solder to the solder wick.This process is required to maintain a repeatable volume of solderwithin the resulting solder joints upon completion of the componentreplacement process. It can be recognized that there are numerousmethods for removing components and residual solder which should notimpose limitations to the present invention. In a third step 214,uncured solder mask is applied to the selective solder mask applicator50. This can be accomplished by subjecting the contacting face 54 of theselective solder mask applicator 50 to a reservoir such as the trough 72of the machined block 70. It can be recognized that there are many othermethods of presenting uncured solder mask to the selective solder maskapplicator 50. In a fourth step 216, the selective solder maskapplicator 40 is located to a desirable location with respect to theplurality of receiving pads 14. This can be accomplished by any numberof methods, including those illustrated in FIG. 6, visual alignment fromthe edge of the applicator 50, and the like. In a fifth step 218,uncured solder mask would be transferred in a desired, selective patternto the PCB 10. In a sixth step 220, the selective solder mask applicator50 is removed from the PCB 10, leaving the desired, selective pattern ofuncured solder mask on the PCB 10. In a seventh step 222, the desired,selective pattern of uncured solder mask is cured using a curing methodrespective to the said solder mask. One example would be to apply heatto a heat curing material such as Loctite™ chip bonder. In an eighthstep 224, a replacement component is attached to the PCB using arespective attachment process. For the example of a BGA, the componentcan be dipped into a predetermined thickness of flux, placed onto theplurality of receiving pads 14, and heated using convection heating tocause solder balls which are pre-attached to the component to becomeliquidous and create electromechanical interconnections between thecomponent and the plurality of electronic pads 14.

[0038] Various changes may be made to the embodiments shown hereinwithout departing from the scope of the present invention which islimited only by the following claims.

What is claimed is:
 1. A method of selectively applying solder mask to aPrinted Circuit Board, the method comprising the steps: registering asolder mask application pattern of a selective solder mask applicator toa pattern of electronic pads respective to a single component of anPrinted Circuit Board, and applying a solder mask material proximate thepattern of electronic pads respective to a single component on anPrinted Circuit Board, wherein the solder mask is applied between theelectronic pads and leaving the electronic pads exposed.
 2. The methodof claim 1 wherein the selective solder mask applicator is a stencilwith a plurality of apertures.
 3. The method of claim 1, the methodfurther comprising the step of applying the solder mask material to thesolder mask applicator, wherein the solder mask applicator is atransferring apparatus.
 4. The method of claim 3, the transferringapparatus comprising a replicated array pattern, wherein the methodfurther comprises the step of reducing the replicated array pattern onthe transferring apparatus to correspond to the pattern of electronicpads respective to the single component on the Printed Circuit Board. 5.The method of claim 3, the transferring apparatus comprising atransferring surface and a pattern of recessions, wherein thetransferring surface is used to transfer the solder mask and the patternof recessions is similar to the pattern of electronic pads on thePrinted Circuit Board to avoid transference of the solder mask.
 6. Themethod of claim 5, wherein the transferring apparatus is of anelastomeric material.
 7. The method of claim 5, wherein the transferringsurface further comprises at least one of texturing and recessions toenhance the transferring process.
 8. A method of selectively applyingsolder mask to a Printed Circuit Board, the method comprising the steps:registering a solder mask application pattern of a selective solder maskapplicator to a pattern of electronic pads respective to a singlecomponent of an Printed Circuit Board, applying a heat curable soldermask material proximate the pattern of electronic pads respective to asingle component on an Printed Circuit Board, wherein the heat curablesolder mask is applied between the electronic pads and leaving theelectronic pads exposed, and applying heat to cure the solder mask. 9.The method of claim 8 wherein the selective solder mask applicator is astencil with a plurality of apertures.
 10. The method of claim 8, themethod further comprising the step of applying the heat curable soldermask material to the solder mask applicator, wherein the solder maskapplicator is a transferring apparatus.
 11. The method of claim 10, thetransferring apparatus comprising a transferring surface and a patternof recessions, wherein the transferring surface is used to transfer theheat curable solder mask material and the pattern of recessions issimilar to the pattern of electronic pads on the Printed Circuit Boardto avoid transference of the heat curable solder mask material.
 12. Themethod of claim 11, wherein the transferring apparatus is of anelastomeric material.
 13. The method of claim 11, wherein thetransferring surface further comprises at least one of texturing andrecessions to enhance the transferring process.
 14. A method ofselectively applying solder mask to a Printed Circuit Board, the methodcomprising the steps: registering a solder mask application pattern of aselective solder mask applicator to a pattern of electronic padsrespective to a single component of an Printed Circuit Board, whereinthe solder mask applicator is at least one of a stencil and a materialtransferring apparatus, and applying a solder mask material proximatethe pattern of electronic pads respective to a single component on anPrinted Circuit Board, wherein the solder mask is applied between theelectronic pads and leaving the electronic pads exposed.
 15. The methodof claim 14, the method further comprising the steps of using amagnifying means to aid in registration of the solder mask applicationpattern of the selective solder mask applicator to the pattern ofelectronic pads respective to the single component of an Printed CircuitBoard.
 16. The method of claim 15 wherein the magnifying means includesa prism.
 17. The method of claim 15 wherein the solder mask material isheat curable and the method further comprising the steps of applyingheat for curing the solder mask material.
 18. The method of claim 14,the solder mask applicator comprising a replicated array pattern,wherein the method further comprises the step of reducing the replicatedarray pattern on the solder mask applicator to correspond to the patternof electronic pads respective to the single component on the PrintedCircuit Board.
 19. The method of claim 14, the method further comprisingthe steps: removing the single component from the Printed Circuit Board,and removing any residual solder from the pattern of electronic padsrespective to the single component of the Printed Circuit Board.
 20. Themethod of claim 19, the method further comprising the step of replacingthe single component onto the Printed Circuit Board.